All stone fruits are drupes, however not all drupes are stone fruits. For instance coconuts, hickory nuts and walnuts are all drupes, with their husks being the exocarp and mesocarp. On the far end of the spectrum is the genus Stryrax, where the exocarp and mesocarp are reduced so much as to be hardly noticeable.

Obviously, this can make categorization of fruits difficult, as the true defining characteristic of a drupe is that the hard stone originates from the ovary wall of the flower.